How do I address mediocre teaching without damaging the relationship?

By targeting the practice, not the person. There's a critical distinction between "your teaching is inadequate" and "the evidence from my visits suggests that students aren't getting enough opportunities to practice independently, and I'd like to work with you on that." The first is a character judgment. The second is a specific, evidence-based observation with an offer of support.

The relationship is actually more likely to be damaged by avoidance than by directness. Teachers know when their practice isn't where it should be. When you don't say anything, they either assume you don't notice (which undermines your credibility) or assume you don't care (which undermines trust). A respectful, prepared, evidence-based conversation communicates that you take their work seriously enough to engage with it honestly.